1. Field of the Invention
The present invention relates to a semiconductor device and a process for manufacturing the same and, more particularly, to a semiconductor substrate having a structure composed of a thin film laminated layer intensively formed with transistor elements and a light valve device having said semiconductor substrate, a liquid crystal layer and an opposed substrate integrated with one another.
2. Description of the Prior Art
In an active matrix device of the prior art, thin film transistors are formed on the surface of an amorphous silicon thin film or a polycrystalline silicon thin film deposited over a glass substrate. The amorphous silicon thin film and the polycrystalline silicon thin film can be easily deposited over the glass substrate by the chemical vapor deposition so that the structure is suited for manufacturing an active matrix liquid crystal display device having a relatively large frame. The transistor elements formed over the amorphous or polycrystalline silicon thin film are generally of the field effect insulated gate type. At present, an active matrix liquid crystal display device using amorphous silicon having an area of about 3 inches to 10 inches is commercially produced. The amorphous silicon thin film is suited for a liquid crystal panel having a large area because it can be formed at a low temperature equal to or less than 350.degree. C. On the other hand, the active matrix liquid crystal display device using the polycrystalline silicon film having a small-sized liquid crystal display panel as wide as about 2 inches is commercially produced at present.
However, the active matrix liquid crystal display device using the amorphous silicon thin film or the polycrystalline silicon thin film of the prior art is suited for a direct view type display device requiring a relatively large frame image plane but is not always suited for miniaturizing the device size and increasing the density of the pixels. In recent years, there has been a growing demand for a microminiature display device or a light valve device having fine and highly dense pixels. This microminiaturized light valve device is utilized as the primary image forming plane of a projection type image device, for example, so that it can be applied to a projection type high-definition TV system. The fine semiconductor manufacturing technology can be used to manufacture a microminiature light valve device which has a pixel size on the order of 10 .mu.m and a total size of about several cm.
However, when the amorphous or polycrystalline silicon thin film of the prior art is used, the transistor elements on the order of sub-microns cannot be formed by applying the fine semiconductor processing technology. Since the amorphous silicon thin film, for example, has a mobility of about 1 cm.sup.2 /Vsec, a driver circuit having the required high speed operation cannot be formed over a common substrate. In case of the polycrystalline silicon thin film, on the other hand, the crystal particle has a size of about several .mu.m, thus creating a problem with regards to the miniaturization of active elements.
On the other hand, the semiconductor device widely utilized has its transistor elements formed on the surface of a single crystal substrate. FIG. 2 is a section showing a semiconductor substrate. Generally speaking, the semiconductor substrate is formed of a single crystal semiconductor substrate 101 made of silicon.
Specifically, the single crystal semiconductor substrate 101 has its surface formed integrally and highly densely with the transistor elements or the like by the impurity diffusion and the film forming process. In the example shown in FIG. 2, the single crystal semiconductor substrate 101 is formed thereover with an insulated gate field effect transistor. The element region to be formed with the transistor is enclosed by a field insulated film 102. The element region is formed with a source region 103 and a drain region 104 by the impurity doping process. Between these source region 103 and drain region 104, there is formed a region 105 for forming the channel of the transistor. This channel forming region 105 is arranged thereover through a gate oxide film 106 with a gate electrode 107. The transistor element composed of those gate electrode 107, the source region 103, the drain region 104 and so on is covered with an inter-layer insulating film 108. Through contract holes formed in the inter-layer insulating film 108, there are arranged a source electrode 109 and a drain electrode 110 for wiring the individual transistors.
The semiconductor substrate made of the silicon single crystal of the prior art, as described above, is superior, having a high speed operation and high density of the transistor elements and so on, to the aforementioned amorphous silicon thin film and polycrystalline silicon thin film.
Since, however, the silicon single crystal substrate is opaque, it cannot be applied as it is to a device such as a light valve device requiring transparency of the substrate.
In recent years, on the other hand, an image projection system has been developed using the light valve device of that kind. This image projection system is desired to have a smaller size, less weight and a finer projection image. To accomplish this, a very high density integrated circuit of the semiconductor device used in the light valve device is required.
Incidentally, the semiconductor device of the prior art is formed with the transistor elements by subjecting one face of the single crystal semiconductor substrate 101 sequentially to the impurity doping process and the film forming process. These processes are always carried out from one face only so that the films are sequentially laminated. As a result, once the lower layer is processed and laminated by an upper layer, it cannot be subjected to an additional treatment any more, thus raising a problem that the step design is restricted in various points.
Although the semiconductor substrate 101 has a surface and back opposed to each other, the semiconductor device is formed by making use of the surface of the semiconductor substrate 101 only. Thus, the wiring of an integrated circuit is concentrated only at the surface while leaving the back unused. As a result, the useable area restricts the wiring density raising a problem in that a far higher density of the integrated circuit cannot be obtained. If the back of the semiconductor substrate could be utilized as the wiring face, the integration density could be effectively doubled. Nevertheless, this two-face wiring has been impossible in the structure of the prior art. In order to raise the integration density, it has also been proposed to wire multiple layers on one face of the semiconductor substrate. With these multi-wiring operations, however, the flatness of the semiconductor substrate surface is degraded resulting in an open defect in step portions or other short-circuit defects.
In the structure of the prior art, the transistor elements are directly integrated on the surface of the single crystal semiconductor substrate. As a result, this single crystal semiconductor substrate is in an integral relation with the transistor elements formed thereover. In other words, the integrated circuit is always supported by the single crystal semiconductor substrate. Depending upon the intended use of the semiconductor device, however, the use of the single crystal semiconductor substrate as the support substrate is frequently improper. Since this support substrate cannot be freely set, the existing structure restrictively limits the flexibility available in the application of the semiconductor device.